(1) Field of the Invention
The present invention relates to an ultrasonic imaging apparatus which functions to display an image of a two-dimensional distribution of a tissue characterization parameter of a tissue, for example, an ultrasonic attenuation coefficient, in a realtime mode.
In the field of the ultrasonic diagnosis, the two-dimensional distribution of the tissue characterization parameters, for example, an ultrasonic attenuation coefficient, an ultrasonic reflection coefficient, an elasticity parameter, are used to diagnose a condition of the tissue. For example, the value of the ultrasonic attenuation coefficient of tissue of a liver suffering from hepatitis is small compared with a normal tissue, and the value of the ultrasonic attenuation coefficient of a fatty liver is large compared with a normal tissue. Since high accuracy is required in ultrasonic diagnosis, the tissue characterization parameters must be determined precisely.
(2) Description of the Related Art
Conventionally, the ultrasonic imaging apparatus which functions to display an image of a two-dimensional distribution of a tissue characterization parameter of a tissue in a realtime mode is disclosed in the Japanese Unexamined Patent Publication No.62-109553, and the U.S. Pat. No.4,836,210 of Shiba et al, dated Jun. 6, 1989.
In ultrasonic imaging apparatuses, a pulsed beam of an ultrasound is generated to inject the beam into a tissue in a certain direction, and ultrasound which is reflected at various depths of the tissue in the same direction are detected. The direction of the pulsed ultrasound is scanned so that a plane is scanned by the pulsed ultrasound during one plane scan cycle. Thus, a two-dimensional distribution of the tissue characterization parameter on the plane (cross section of the tissue) is obtained. The above injection and detection of the ultrasound is carried out through an ultrasonic transducer array. The operator (doctor) applies a probe containing the transducer array on a skin of a human body to diagnose the condition of the tissue under the skin.
However, the obtained values of the above tissue characterization parameters fluctuate due to subtle movements of the probe or the tissue which is subject to the diagnosis. Further, for example, the values of the ultrasonic attenuation coefficient, which are obtained as above, greatly vary when a structure which is different from the tissue which is to be diagnosed, e.g., a vascular tract, exists in the scanned cross section. Generally, it is considered that the obtained values of the tissue characterization parameters are significant only when the obtained values are stable. Thus, generally, the values of the tissue characterization parameters must be obtained on a cross section of the tissue where the obtained values are stable. Nevertheless, conventionally, it is difficult for the operator to determine whether or not an obtained value of a tissue characterization parameter is stable. Therefore, in conventional ultrasonic imaging apparatuses, the credibility of the obtained values of the tissue characterization parameters is low.